feat: add PDF support via mupdf WASM

[ndemarchis]

Summary

• Adds PDF → text/markdown conversion using the mupdf npm package (AGPL-3.0, same license as VERT), running entirely client-side in a web worker
• Adds generic converter chaining so formats can be converted through intermediate steps (e.g. PDF → md → docx via mupdf + pandoc)
• The mupdf WASM binary is not committed — a postinstall script copies it from node_modules

Supported conversions

• PDF → md, txt (direct via mupdf)
• PDF → docx, html, rtf, rst, epub, odt, docbook, json (chained through pandoc)